Add habitat.filters.* pre-filters for removing nulls:

[danielrichman]

Some people fly payloads that transmit 0x00 while waiting for sensors. UKHAS will refuse to even contemplate parsing them.

Idea 1: habitat.filters.remove_nulls: config could be a prefix (i.e., only fix sentences that begin with '$$CALLSIGN'); just strips out all nulls. Could configure it with the positions the nulls should be in.

Idea 2: a pre-filter that just applies a regexp. Infact, this could even be useful as an intermediate filter.